One such reengagement downshift control, executed in a coast lockup state, has been disclosed and proposed in Patent document 1.
The aforementioned conventional reengagement downshift control technique aims at keeping a lockup state (a coast lockup state), in which an input element and an output element are directly coupled together within a torque converter, as long as possible, so that engine speed can be held at speeds higher than the fuel-cutoff recovery rotational speed (the fuel-injection restart rotational speed), in order to improve fuel economy by prolonging the engine fuel-cutoff time (i.e., the fuel-injection stop time) under an engine no-load condition. Hence, the engagement hydraulic pressure of a reengagement-downshift speed-change friction element is determined on the assumption that the torque converter is in the previously-discussed coast lockup state.
However, suppose that, of these reengagement-downshift speed-change friction elements, the engagement hydraulic pressure of an engagement-side speed-change friction element to be switched from its release state to its engagement state is determined on the assumption that the torque converter is in the coast lockup state. In such a case, there is the following concern.
That is, it may become an operating condition in which the lockup state of the torque converter should be released, even though the same engine no-load condition remains unchanged after a downshift followed by switching (reengagement) between friction elements in a coast lockup state has been started.
In such a case, according to the conventional coasting reengagement downshift control, the downshift will progress based on the engagement hydraulic pressure of an engagement-side speed-change friction element, which pressure is determined on the assumption that the torque converter is in a coast lockup state.
However, with the torque converter whose lockup state has been practically released, a transmitted torque (coast torque) of the automatic transmission becomes less than a transmitted torque (coast torque) in the coast lockup state. Thus, the engagement hydraulic pressure of the engagement-side speed-change friction element tends to become excessively high as compared to the transmitted torque of the automatic transmission.
For the reasons discussed above, there is an increased tendency for the drawing-in phenomenon of torque to occur at the beginning of the inertia phase during reengagement-downshifting. Also, there is an increased tendency for the pop-up phenomenon of torque to occur at the end of the inertia phase during reengagement-downshifting. Anyway, there is a problem that the shift quality of the coasting reengagement downshift is deteriorated.